Cover layer for electrical conductors or semiconductors

ABSTRACT

The invention relates to a protective coating on at least one electrical conductor and/or semiconductor, especially an electric sensor, in which a porous first protective coating is applied directly to the surface of the electrical conductor and/or semiconductor and a gas-tight second protective coating is applied at least to regions of the first protective coating.

BACKGROUND OF THE INVENTION

The present invention relates to a cover layer on at least oneelectrical conductor and/or semiconductor, especially of an electricsensor.

Cover layers for electrical conductors are known (DE-23 04 464 C2). Theyconsist for example of barium-aluminum silicates, applied using theusual methods employed in ceramics technology. Such coatings aregenerally gas-tight cover layers.

In addition, porous cover layers are also known that consist for exampleof a metal, a metal alloy, an oxide, or a mixed oxide such as magnesiumspinel, carbides, borides, nitrites of transition metals, or silicateminerals such as high-melting sintered glass or fireproof ceramicmaterials for example, which can also be applied as raw materials ormixtures of raw materials such as kaolin or talc, possibly with theaddition of fluxes such as feldspar, nepheline syenite, or wollastonite,and then sintered.

In known protective layers, the gas-tight cover layer is applied to asolid electrolyte, while the porous first protective layer is locatedabove the electrodes.

It has been found that when electrical conductors are covered withgas-tight cover layers, ions such as calcium, sodium, or oxygen ionsmigrate under the influence of the electrical voltages applied to theconductors, especially at higher temperatures, causing damage in thefirst protective layer and consequently in the conductors as well. Inaddition, discharges of oxygen ions can also release oxygen bubbles.

SUMMARY OF THE INVENTION

The goal of the present invention is to create cover layers in which ionmigration is reduced in normal operating states.

In the cover layers according to the invention, a porous firstprotective layer is located directly on the surface of the electricalconductor and/or semiconductor and a gas-tight second protective layeris located at least areawise on the porous first protective layer.

Surprisingly, oxygen ion migration can be reduced by applying the porousfirst protective layer beneath the gas-tight second protective layer onthe surface of the electrical conductor or semiconductor. As a result,the migration of oxygen ions between conductors with differentpotentials, and hence their harmful influence, can be reduced. In thisconnection it is theoretically immaterial whether the porous firstprotective layer is made with open or closed pores, although theopen-pored state is more advantageous. If on the other hand the porousfirst protective layer is to be used to cover the electrodes and/orgas-sensitive layers of electrical sensors that measure gas, open-poredporosity is essential. The porous first protective layer contains 20 to60 parts by weight of silicon dioxide, 28 to 75 parts by weight ofaluminum oxide, and 1 to 27 parts by weight of barium oxide.

The gas-tight second protective layer contains 20 to 56 parts by weightof silicon dioxide, 28 to 75 parts by weight of aluminum oxide, and 2 to55 parts by weight of barium oxide.

The first porous protective layer preferably contains 28 to 50 parts byweight of silicon dioxide and 30 to 65 parts by weight of aluminumoxide. The addition of barium oxide preferably is on the order of 2 to20 parts by weight.

The gas-tight second protective layer preferably contains 28 to 50 partsby weight of silicon oxide and 30 to 65 parts by weight of aluminumoxide. The barium oxide content is on the order of 4 to 50 parts byweight of barium oxide. The porosity of the two protective layers can becontrolled by the barium oxide content. The gas-tight second protectivelayer has a barium oxide content that is generally 2 to 8.5 timesgreater than that of the porous first protective layer.

The gas-tight second protective layer especially preferably contains 30to 45 parts by weight of silicon dioxide, and especially 35 to 40 partsby weight of silicon dioxide. The aluminum oxide content of thegas-tight second protective layer is advantageously in the range from30.0 to 65.0 parts by weight, especially preferably in the range from40.0 to 60 parts by weight, and especially in the range from 47.0 to53.0 parts by weight. The content of barium oxide is preferably in therange from 4 to 50 parts by weight, especially preferably in the rangefrom 5 to 45 parts by weight, especially in the range from 10 to 30parts by weight, and very specially in the range from 12 to 25 parts byweight.

The gas-tight second protective layer advantageously has a barium oxidecontent three to seven times greater than that of the porous firstprotective layer. Very especially preferably, the barium oxide contentin the gas-tight second protective layer is preferably about 3.5 timesgreater.

The entire surface of the electrical conductor or semiconductor can becovered by the porous first protective layer, with the surfaces of theconductors or sensitive layers, for example temperature-sensitiveconductors, semiconductors, or electrodes, e.g. measuring or referenceelectrodes, being covered by the porous first protective layer. Thesurfaces of the electrodes can also be kept clear of the porous firstprotective layer, however. These protective layers can be applied by anyconventional application methods such as for example screen printing,dipping, or flame spraying methods, or by thin-film techniques, such asevaporation or sputtering. Especially advantageously, the protectivelayers are applied by coating methods using thick-film technology, suchas screen printing for example. The gas-tight second protective layer islocated on the porous first protective layer and can completely coverthe latter, except for the electrode areas and the sensitive layers.

It has been found to be advantageous to adjust the content of silicondioxide and aluminum oxide at least partially by adding kaolin thatcontains both aluminum oxide and silicon dioxide. The fine adjustment ofthe desired recipe is then performed by adding aluminum oxide, silicondioxide, and barium oxide. The porosity of the protective layers isadjusted in particular by the barium oxide content. An increase inbarium content within the limits specified above lowers the meltingpoint and thus improves the sintering behavior. The lower the meltingpoint, the less porous the mixtures will be after sintering. Theprotective layers are generally not heated to their melting points, butare sintered below the melting point, and form a glassy or crystallineoxide mixture of components.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe drawing and the examples, wherein

FIG. 1 shows a conductor with cover layers in a partially cutawaysection; and

FIG. 2 is a section II--II according to FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An electrical sensor 15 is shown in FIG. 1 in a cutaway section. Anelectrical conductor 12 and 13 (FIG. 2) is mounted on a substrate 10 andextends to an electrode 18. A porous first protective layer 14 isapplied on conductor 12, said layer also covering electrode 18 andextending on the sides of conductors 12, 13 up to substrate 10 (FIG. 2).A second protective layer 16 that is gas-tight but does not coverelectrode 18 is applied at the remaining locations to the porous firstprotective layer 14 and also borders substrate 10 (FIG. 2).

EXAMPLE 1

Manufacture of the starting material for a porous layer

13.3 g of kaolin, 1.6 g of silicon dioxide, 5.9 g of aluminum oxide, and1.0 g of barium carbonate were homogenized in a planetary ball mill bygrinding. The mixture was then placed in a 300 ml agate beaker, reactedwith 30 ml ethanol, and homogenized for eight hours after adding eightagate balls. The homogenized mixture was then calcined for ten hours at1000° C. in a normal furnace atmosphere. The furnace was heated fromroom temperature to calcination temperature at a rate of 10K per minuteand then allowed to cool to room temperature.

The powder produced by calcination was homogenized by grinding in aplanetary ball mill. The powder was placed in a 250 ml agate beaker,reacted with 30 ml ethanol, and ground for twelve hours after addingeight agate balls. After grinding, the homogenized mixture was dried ina drying cabinet.

The dried powder was added in batches of 10 g to an 80 ml agate beaker,reacted with 6.5 g of a screen-printing medium made of terpene oil andethyl cellulose, and then, after adding seven agate balls, homogenizedfor four hours in a planetary ball mill.

EXAMPLE 2

Screen Printing and Firing

The screen-printing paste manufactured according to Example 1 wasapplied by screen printing to an electrical conductor. Then thefollowing sintering procedure was used:

1. Heat to 400° C. at a rate of 10K per minute;

2. Hold for 30 minutes at 400° C.;

3. Heat to 1330° C. at a rate of 5K per minute;

4. Hold for 60 minutes at 1330° C. (sintering);

5. Cool to 25° C. at a rate of 10K per minute.

In this fashion, a porous first protective layer was obtained on theconductor.

EXAMPLE 3

Production of a Porous Layer

In accordance with Example 1, a mixture of 13.6 g of kaolin, 1.8 g ofsilicon dioxide, 4.5 g of aluminum oxide, and 4.9 g of barium carbonatewere homogenized and further processed into a screen-printing paste.

The printing of a conductor and the sintering of the porous firstprotective layer were performed according to Example 2 at a sinteringtemperature of 1280° C.

EXAMPLE 4

Production of Starting Material for a Gas-Tight Cover Layer

11.0 g of aluminum oxide, 8.0 g silicon dioxide, and 4.0 g bariumcarbonate were homogenized as in Example 1, calcined, and processed toform a screen-printing paste.

EXAMPLE 5

Production of starting material for a gas-tight layer

9.7 g of aluminum oxide, 6.8 g of silicon dioxide, and 8.4 g of bariumcarbonate were homogenized as in Example 1, calcined, and processed toform a screen-printing paste.

EXAMPLE 6

The screen printing pastes according to Examples 4 and 5 were appliedaccording to Example 2 by screen printing to a porous protective layerobtained according to Example 2 and sintered as described in Examples 2and 3.

By a combination of the porous layers according to Example 1 or 3, aswell as the gas-tight layers according to Example 4 or 5, a two-layersystem was obtained that has a porous first protective layer on theconductor and a gas-tight second protective layer on top of that.

We claim:
 1. Cover layer on at least one electrical conductor and/orsemiconductor, characterized in that a porous first protective layer(14) is applied as a cover layer directly to the surface of anelectrical conductor and/or semiconductor (12), and a gas-tight secondprotective layer (16) is provided on the porous first protective layer(14), and characterized in that the porous first protective layer (14)has the following composition:20-60 parts by weight of silicon dioxide28-75 parts by weight of aluminum oxide 1-27 parts by weight of bariumoxide.
 2. Cover layer according to claim 1, further comprising anelectrode (18) in contact with the electrical conductor and/orsemiconductor (12), characterized in that the gas-tight secondprotective layer (16) does not cover the electrode (18).
 3. Cover layeraccording to claims 2 or 3, characterized in that the porous firstprotective layer (14) contains 30 to 65 parts by weight aluminum oxide.4. Cover layer according to claim 3, characterized in that the porousfirst protective layer (14) contains 40 to 60 parts by weight aluminumoxide.
 5. Cover layer according to claim 4, characterized in that theporous first protective layer (14) contains 54.0 to 59.0 parts by weightaluminum oxide.
 6. Cover layer on at least one electrical conductorand/or semiconductor, characterized in that a porous first protectivelayer (14) is applied as a cover layer directly to the surface of anelectrical conductor and/or semiconductor (12), and a gas-tight secondprotective layer (16) is provided on the porous first protective layer(14), and characterized in that the gas-tight second protective layer(16) has the following composition:20-56 parts by weight of silicondioxide 28-75 parts by weight of aluminum oxide 2-55 parts by weight ofbarium oxideprovided that the amount by weight of barium oxide in thegas-tight second protective layer (16) is approximately 2 to 8.5 timesgreater than that in the porous first protective layer (14).
 7. Coverlayer according to one of claims 1 or 6, characterized in that thesilicon dioxide and aluminum oxide content is at least partiallyestablished by addition of kaolin.
 8. Cover layer according to claim 6,wherein the gas-tight second protective layer (16) is not in contactwith the electrical conductor and/or semiconductor (12).
 9. Cover layeron at least one electrical conductor and/or semiconductor, characterizedin that a porous first protective layer (14) is applied as a cover layerdirectly to the surface of an electrical conductor and/or semiconductor(12), and a gas-tight second protective layer (16) is provided on theporous first protective layer (14), and characterized in that thegas-tight second protective layer (16) contains from 28 to 50 parts byweight of silicon dioxide.
 10. Cover layer according to claim 5,characterized in that the gas-tight second protective layer (16)contains from 30 to 45 parts by weight of silicon dioxide.
 11. Coverlayer according to claim 10, characterized in that the gas-tight secondprotective layer (16) contains from 35 to 40 parts by weight of silicondioxide.
 12. Cover layer on at least one electrical conductor and/orsemiconductor, characterized in that a porous first protective layer(14) is applied as a cover layer directly to the surface of anelectrical conductor and/or semiconductor (12), and a gas-tight secondprotective layer (16) is provided on the porous first protective layer(14), and characterized in that the gas-tight second protective layer(16) contains from 30 to 65 parts by weight of aluminum oxide.
 13. Coverlayer according to claim 12, characterized in that the gas-tight secondprotective layer (16) contains from 40 to 60 parts by weight of aluminumoxide.
 14. Cover layer according to claim 13, characterized in that thegas-tight second protective layer (16) contains from 47.0 to 53.0 partsby weight of aluminum oxide.
 15. Cover layer on at least one electricalconductor and/or semiconductor, characterized in that a porous firstprotective layer (14) is applied as a cover layer directly to thesurface of an electrical conductor and/or semiconductor (12), and agas-tight second protective layer (16) is provided on the porous firstprotective layer (14), and characterized in that the gas-tight secondprotective layer (16) contains from 4 to 50 parts by weight of bariumoxide.
 16. Cover layer according to claim 15, characterized in that thegas-tight second protective layer (16) contains from 5 to 45 parts byweight of barium oxide.
 17. Cover layer according to claim 16,characterized in that the gas-tight second protective layer (16)contains from 10 to 30 parts by weight of barium oxide.
 18. Cover layeraccording to claim 17, characterized in that the gas-tight secondprotective layer (16) contains from 12 to 25 parts by weight of bariumoxide.
 19. Cover layer on at least one electrical conductor and/orsemiconductor, characterized in that a porous first protective layer(14) is applied as a cover layer directly to the surface of anelectrical conductor and/or semiconductor (12), and a gas-tight secondprotective layer (16) is provided on the porous first protective layer(14), and characterized in that the porous first protective layer (14)contains from 28 to 50 parts by weight of silicon dioxide.
 20. Coverlayer according to claim 19, characterized in that the porous firstprotective layer (14) contains from 30 to 45 parts by weight of silicondioxide.
 21. Cover layer according to claim 20, characterized in thatthe porous first protective layer (14) contains from 35 to 40 parts byweight of silicon dioxide.
 22. Cover layer on at least one electricalconductor and/or semiconductor, characterized in that a porous firstprotective layer (14) is applied as a cover layer directly to thesurface of an electrical conductor and/or semiconductor (12), and agas-tight second protective layer (16) is provided on the porous firstprotective layer (14), and characterized in that the porous firstprotective layer (14) contains 2 to 20 parts by weight barium oxide. 23.Cover layer according to claim 22, characterized in that the porousfirst protective layer (14) contains 2.5 to 15 parts by weight bariumoxide.
 24. Cover layer according to claim 23, characterized in that theporous first protective layer (14) contains 3 to 10 parts by weightbarium oxide.
 25. Cover layer on at least one electrical conductorand/or semiconductor, characterized in that a porous first protectivelayer (14) is applied as a cover layer directly to the surface of anelectrical conductor and/or semiconductor (12), and a gas-tight secondprotective layer (16) is provided on the porous first protective layer(14), and characterized in that the barium oxide content of thegas-tight second protective layer (16) is 3 to 7 times larger than thatof the porous first protective layer (14).
 26. Cover layer according toclaim 25, characterized in that the barium oxide content of thegas-tight second protective layer (16) is approximately 3.5 times largerthan that of the porous first protective layer (14).
 27. Cover layer onat least one electrical conductor and/or semiconductor, characterized inthat a porous first protective layer (14) is applied as a cover layerdirectly to the surface of an electrical conductor and/or semiconductor(12), and a gas-tight second protective layer (16) is provided on theporous first protective layer (14), and characterized in that theelectrical conductor (12, 13) applied to a substrate (10) is fullycovered by the porous first protective layer (14) and the latter isfully covered by the gas-tight second protective layer (16).
 28. Anelectrical device, comprising:an electrical conductor and/orsemiconductor; a porous first protective cover layer provided directlyover a surface of the electrical conductor and/or semiconductor; and agas-tight second protective cover layer provided over the porous firstprotective cover layer, wherein the gas-tight second protective coverlayer is not in contact with the electrical conductor and/orsemiconductor, being separated therefrom by the porous first protectivecover layer, wherein the porous first protective cover layer and thegas-type second protective cover layer each comprises silicon dioxide,aluminum oxide and barium oxide, the gas-tight second protective coverlayer containing more barium oxide than the porous first protectivecover layer, and wherein the porous first protective cover layercomprises 20-60 parts by weight silicon dioxide, 28-75 parts by weightaluminum oxide and 1-27 parts by weight barium oxide.
 29. An electricaldevice according to claim 28, wherein the electrical device is anelectrical sensor and further comprises a substrate over which theelectrical conductor and/or semiconductor is provided.
 30. An electricaldevice according to claim 29, further comprising an electrode providedover the substrate and in contact with the electrical conductor and/orsemiconductor, wherein the gas-tight second protective cover layer doesnot cover the electrode.
 31. An electrical device according to claim 30,wherein the porous first protective cover layer covers the electrode.32. An electrical device according to claim 31, wherein the gas-tightprotective cover layer covers all of the porous first protective coverlayer except in an area over the electrode.
 33. An electrical deviceaccording to claim 31, wherein the porous first protective cover layerhas open-pored porosity.
 34. An electrical device according to claim 33,wherein the electrode is a measuring or reference electrode.
 35. Anelectrical device according to claim 28, wherein the gas-tight secondprotective cover layer comprises 20-56 parts by weight silicon dioxide,28-75 parts by weight aluminum oxide and 2-55 parts by weight bariumoxide.
 36. An electrical device according to claim 35, wherein theamount by weight of barium oxide in the gas-tight second protectivecover layer is 2 to 8.5 times greater than in the porous firstprotective cover layer.
 37. An electrical device, comprising:anelectrical conductor and/or semiconductor; a porous first protectivecover layer provided directly over a surface of the electrical conductorand/or semiconductor; and a gas-tight second protective cover layerprovided over the porous first protective cover layer, wherein thegas-tight second protective cover layer is not in contact with theelectrical conductor and/or semiconductor, being separated therefrom bythe porous first protective cover layer, wherein the porous firstprotective cover layer and the gas-type second protective cover layereach comprises silicon dioxide, aluminum oxide and barium oxide, thegas-tight second protective cover layer containing more barium oxidethan the porous first protective cover layer, and wherein the gas-tightsecond protective cover layer comprises 20-56 parts by weight silicondioxide, 28-75 parts by weight aluminum oxide and 2-55 parts by weightbarium oxide.
 38. An electrical device according to claim 37, whereinthe electrical device is an electrical sensor and further comprises asubstrate over which the electrical conductor and/or semiconductor isprovided.
 39. An electrical device according to claim 38, furthercomprising an electrode provided over the substrate and in contact withthe electrical conductor and/or semiconductor, wherein the gas-tightsecond protective cover layer does not cover the electrode.
 40. Anelectrical device according to claim 39, wherein the porous firstprotective cover layer covers the electrode.
 41. An electrical deviceaccording to claim 40, wherein the gas-tight protective cover layercovers all of the porous first protective cover layer except in an areaover the electrode.